Culprit lesions in acute coronary syndrome [acute myocardial infarction (AMI) and unstable angina pectoris (UAP)] were examined angiographically in 222 patients who had previously undergone coronary angiography (CAG). The observation period lasted 5 years after primary CAG in medically treated patients (group M, 127 cases) and after final follow-up CAG in patients treated by percutaneous transluminal coronary angioplasty (PTCA) (group B, 95 cases). There were 33 AMIs, including 5 deaths (22/127, 17.3%, in group M vs 11/95, 11.6%, in group B; p<0.01) and 189 UAPs (105/127, 82.7%, in group M vs 84/95, 88.4%, in group B; NS). High-grade stenoses (>75%) were found in 76 (59.8%) patients in group M, of which 41 lesions (54%) resulted in acute coronary syndromes (ACSs). In group M, ACSs resulted from insignificant stenosis (< or =50%) in 67 (53%) patients and from significant stenosis (>50%) in 60 (47%) patients. In group B, ACSs resulted from insignificant stenosis in 78 (82%) patients and from significant stenosis in 17 (18%) patients. Out of 95 PTCA sites, high-grade restenosis occurred in 3 lesions and ACSs (2 AMI, 14 UAP) in 16 (16.8%). We conclude that ACSs are more likely to develop from insignificant lesions than from significant lesions. High-grade stenoses are prone to become occlusive lesions and PTCA reduces this potential risk. Most target sites of PTCA that escaped restenosis were stable in the long term.